I wouldn't mind a sequel at all, provided that it was about someone else than Manny. The world of GF is so deep and intriguing that it's a downright shame no other stories have been set there. We only get to see a small portion of the Land of the Dead and there's so much potential for more. Even if you decided to use the exact same locations, there's still room for a great story.
I think someone started a fanfic about one of Hector's goons and what happened to him after Manny defeated his boss. I think that could be an excellent starting point for a game because it would let us see things from a different perspective and there's lots of potential for an interesting story and character development. Why was this person working for Hector? Maybe he just died and just wanted a job, not knowing what he was getting himself into. What will he do now that Hector is down? Will some other crime boss try to step up and take Hector's place? Will the corruption come back? What will the LSA do? Include a cast of new characters, cameos from some old faces (Eva, Maximino, Membrillo, Toto etc.), some plots twists and an intriguing plot about crime and corruption and you've got another brilliant game at your hands.
Even if it sucked, I don't see how it could ever ruin the brilliance of the original in any way.